In recent years, different steer-by-wire steering systems (also referred to herein, in the following, as SBW steering systems) for motor vehicles have been developed, in which a mechanical connection is no longer present between a steering wheel, which is actuated by a vehicle driver, and the steered wheels of the vehicle.
Newer concepts of SBW steering systems implement an electrically operable steering actuator for each steerable vehicle wheel, in order to enable said steering actuator to adjust the steering angle of the particular steerable wheel by application of a steering torque. This results in advantages, such as, for example, an ideal Ackermann angle under all driving conditions, which, in turn, results in less tire wear and a better tire adherence during cornering. In addition, greater play in terms of the configuration results for the placement of the individual steering actuators as compared to a conventional rack-and-pinion steering. A steering system of this type is described in DE 10 2008 039 547 A1, for example.
In order to make it possible to continue ensuring a certain steerability of the vehicle even in the event of a failure of a steering actuator, different components of the SBW steering system can be redundantly designed, i.e., in duplicate, for example. This relates not only to hardware components, such as, for example, the steering actuators themselves, an electronic control unit (ECU) which controls the steering actuators, steering torque sensors and steering angle sensors, communication paths between the individual components and the like, but also to the software which controls the SBW steering system.
In the event of a failure of a steering actuator, the steering actuator can be designed in such a way that it prevents or freely permits a rotation of the vehicle wheel. Alternatively, the steering actuator can also be designed in such a way that it permits a rotation of the steerable vehicle wheel only if a certain steering torque has been exceeded. The steering actuator which is still functioning can then, inter alia, take over the steering task of the failed steering actuator, in order to be capable of at least safely bringing the vehicle to a standstill.
In this way, the aforementioned document DE 10 2008 039 547 A1 provides, for example, that a pair of steering actuators is mechanically interconnected via a safety cable, and so, in the event of a malfunction of one steering actuator, a portion of the steering torque generated by the other, non-faulty steering actuator can be transferred to the faulty steering actuator via the safety cable, in order to allow the vehicle wheel connected to the faulty steering actuator to still be steered at least to a certain extent.
A steering system, in which the steerable front wheels and the steerable rear wheels of an automatically controlled vehicle are each steered by a separate steering actuator (one for the front axle and one for the rear axle of the vehicle), is disclosed in WO 2008/129275 A1. In order to nevertheless allow for the continued steering of the front and rear wheels of the vehicle in the event of a failure of one steering actuator, the two steering actuators can be mechanically, hydraulically, or pneumatically coupled to each other via a coupling, and so the steering actuator which is still functioning can take over the steering task of the failed steering actuator. In one exemplary embodiment, the steering actuators are designed as electric motors, wherein an output shaft of the electric motor extends to two sides of the particular steering actuator and one end of the output shaft is connected to a steering mechanism for steering the front and rear wheels, and the other end of the output shaft is connected to a coupling, via which the two steering actuators or their corresponding output shafts can be coupled to each other.
In addition, document U.S. Pat. No. 7,021,416 B2 makes known a steer-by-wire steering system which comprises a mechanical safety connection in the form of a flexible shaft between a hand grip device, which can be operated by a vehicle driver, and a rack-and-pinion steering mechanism for steering two steerable vehicle wheels. A mechanical coupling of the flexible shaft to the rack-and-pinion steering mechanism can be established with the aid of an electromagnetic coupling in the event of an electrical power failure of the SBW steering system, and so the vehicle wheels can be steered via the hand grip device in this case.
In addition, in the case of a steer-by-wire steering system for a vehicle disclosed in document US 2008/0149412 A1, a mechanical coupling can be established between a steering wheel and a steering mechanism with the aid of an electromagnetic coupling in order to steer the steerable vehicle wheels in the event of a malfunction of the SBW steering system.
Against this background, the problem addressed by the present invention is that of providing an improved steering system, in particular a steer-by-wire steering system, for a vehicle, which ensures the steerability of the vehicle, at least to a certain extent, even in the event of a technical fault in the steering system. In addition, the steering system should be designed to be extremely compact.
This problem is solved by the steering system of the present invention. It should be noted that the features mentioned individually in the following description can be combined with one another in any technically reasonable manner and can reveal further embodiments of the invention. The description additionally characterizes and specifies the invention in particular in connection with the figures.